Splitted side-effecting logic from pure computation in `ScalaCompletions`

Created two nested method `typeCompletionsAt` and `scopeCompletionsAt` to
clearly separate the side-effecting logic from the pure computation in
`ScalaCompletions`.
Don't add parameter list on import completion for module's methods

Also, added ignored test for Re #1001919

Fixes #1001125
Added explicit 'override' in `ScalaCompletions`
Merge pull request #568 from dotta/issue/import-completion-for-module-methods

Issue/import completion for module methods